1. Technical Field
The present invention relates generally to the field of semiconductor fabrication and in particular to photolithographic processes used in the manufacture of semiconductor devices. Still more particular, the present invention relates to a method and apparatus for reducing degradation in the contrast of resist images produced from a mask image.
2. Description of the Related Art
Microcircuit fabrication requires that precisely controlled quantities of impurities be introduced into very small regions of a substrate. These regions are subsequently interconnected to created components and very large scale integration (VLSI) circuits. The patterns that define these regions are created by lithographic processes. Typically, photoresist materials are spun onto a wafer substrate. Then, the photoresist is selectively exposed to radiation, such as ultraviolet light, electrons or x-rays. An exposure tool and a mask are used to cause the desired exposure of the photoresist. The patterns in the resist are formed when the wafer under goes a development step. The areas of photoresist remaining after development of the photoresist protect the covered regions of the substrate during introduction of impurities or during etching of exposed regions of the substrate.
In the art of deep ultraviolet microlithography, much demand as technology develops is placed upon increased resolution, tighter placement of features in proximity to one another, and smaller but well-defined features. In order to continue meet increasing demands, many methods and devices have been developed and tested. One such concept is the control of images exposed onto a resist layer by enhancing the contrast of the said images.
In the process of patterning an image on a layer of resist on a wafer, light passes through openings on a mask, then through a lens chamber, and finally onto the wafer that is coated with resist. A xe2x80x9cmaskxe2x80x9d is a pattern tool, which contains patterns that can be transferred to an entire wafer or to another mask in one exposure. At the resist level, portions of the resist that coincide with the image pattern are exposed and then developed to reproduce the same original image in a greatly reduced form. When light passes though the mask, sub harmonics are introduced into the intensity distribution of the aerial image. These sub harmonics cause the aerial image formed at the resist layer to be an inexact replica of the mask image. The resist image formed from the raw aerial image tends to have degraded contrast using presently available processes and equipment due to coinciding harmonics that are transmitted in addition to the desired zeroeth order element. Therefore, the same sub harmonics also limit the maximum resolution allowed in the final image. In order to reduce the distortions, previous methods improved the contrast by relying on the use of an extra layer that is formulated specifically for contrast enhancement coated on top of the resist layer.
However, such methods lengthen the processing of semiconductors by requiring additional process steps to place a coat of a contrast enhancement layer on top of the resist layer.
Therefore it would be advantageous to have a system and a method for enhancing contrast with minimal additional process steps.
The present invention provides an intensity filter for deep ultraviolet lithography for enhancing contrast. The intensity filter filters light having various intensities. The intensity filter includes a first material and a second material in which these two materials interface in such a way that only specific intensities are passed through. The first material is non-linear in nature and has a refractive index that changes at high intensities, but has a constant refractive index substantially equivalent to the second material at a selected intensity. The second material has a constant refractive index regardless of varying levels of intensity, at intensities lower than a specific minimum threshold. The filter also may include a coating that will phase shift the exiting filtered light 180-degrees.